This invention relates to a nickel-hydrogen secondary battery.
As a nickel-hydrogen secondary battery where a hydrogen-absorbing alloy is employed as a negative electrode, those provided with a negative electrode comprising an LaNi.sub.5 -based alloy containing CaCu.sub.5 type structure phase as a main phase, or an alloy having a Laves type structure and containing constituent elements such as Ti, Zr, V and Ni have been conventionally put into practical use. In particular, most of secondary batteries produced at present is occupied by a nickel-hydrogen secondary battery employing an LaNi.sub.5 -based alloy, and this nickel-hydrogen secondary battery has been extensively employed. However, since the hydrogen absorption capacity of the LaNi.sub.5 -based alloy is basically in the ratio of 1:1 (hydrogen atom:metal element), it is very difficult with this LaNi.sub.5 -based alloy to enhance the hydrogen absorption over the aforementioned ratio. When this hydrogen absorption capacity of the LaNi.sub.5 -based alloy is converted into an electrochemical capacity, it corresponds to about 370 mAh/g. However, since the LaNi.sub.5 -based alloy which is currently employed in the nickel-hydrogen secondary battery already indicates a hydrogen absorption capacity of as high as about 330 mAh/g, a more prominent increase in capacity density can be hardly expected as far as this LaNi.sub.5 -based alloy is concerned.
On the other hand, the Laves type alloy is known as being capable of absorbing hydrogen in the ratio of 1:1 or more (hydrogen atom:metal element). Therefore, it is theoretically possible with this Laves type alloy to enhance the capacity density of hydrogen absorption. However, this Laves type alloy is accompanied with problems that a stable oxide film tends to be formed on the surface of the alloy thereby making it impossible to sufficiently utilize the alloy, that it takes a relatively long time for initially activating the alloy, that it is difficult to obtain a sufficient charge/discharge efficiency, and that it is difficult to achieve a high capacity density in concurrent with other desired battery properties, thus making it difficult to extensibly utilize the Laves type alloy.
Meanwhile, a newly developed hydrogen-absorbing alloy comprising magnesium, nickel and a rare earth element is advantageous over the LaNi.sub.5 -based alloy and Laves type alloy in that the absorption capacity density thereof both per volume and per weight is higher than that of the LaNi.sub.5 -based alloy, that the activation velocity thereof is higher than that of the Laves type alloy, and that it is possible to obtain an excellent charge/discharge efficiency as compared with the Laves type alloy. Therefore, it is possible with this hydrogen-absorbing alloy to realize a secondary battery exhibiting a higher capacity as compared with a secondary battery employing the LaNi.sub.5 -based alloy and also exhibiting a more excellent charge/discharge efficiency as compared with a secondary battery employing the Laves type alloy.
However, a nickel-hydrogen secondary battery provided with a negative electrode comprising a hydrogen-absorbing alloy containing magnesium, nickel and a rare earth element is also accompanied with the problems that, if the quantity of electrolyte per capacity of battery and the quantity of hydrogen-absorbing alloy of the negative electrode are made identical with those of the battery employing the LaNi.sub.5 -based alloy, it would result not only in a failure to achieve an enhancement in capacity of the battery, but also in an increase in inner pressure due to an insufficient gas recombination reaction at the occasion of overcharging at the negative electrode or in a decrease in discharge capacity at low temperatures.
Meantime, Japanese Patent Unexamined Publication No. H/5-343060 discloses a battery provided with a negative electrode containing a hydrogen-absorbing alloy at packing density of 5.2 to 5.7 g/cm.sup.3. This publication recommends the employment of a Ti--Ni type hydrogen-absorbing alloy as the hydrogen-absorbing alloy.
Further, Japanese Patent Unexamined Publication H/6-223868 discloses an alkaline secondary battery provided with a case housing therein an electrode group consisting of a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode, wherein the electrode group is constructed such that the dimensional relationship between the total thickness B of the layers constituting the electrode group and the width A of the inner dimension of the case as taken along the thickness-wise direction of the electrode group becomes 1 to less than 1.
Furthermore, International Re-publication No. WO97/03213 discloses a electrode comprising a hydrogen-absorbing alloy having a specific antiphase boundary and a composition represented by the following general formula (1). This hydrogen-absorbing alloy is mentioned therein as having a LaNi.sub.5 crystal structure. EQU (R.sub.1-x L.sub.x) (Ni.sub.1-y M.sub.y).sub.z (1)
wherein R is La, Ce, Pr, Nd or mixture elements thereof; L is Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Mg, Ca or mixture elements thereof; M is Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, Si, V, Cr, Nb, Hf, Ta, W, B, C or mixture elements thereof; and x, y and z are atomic ratio individually defined as 0.05.ltoreq.x.ltoreq.0.4, 0.ltoreq.y.ltoreq.0.5, and 3.0.ltoreq.z&lt;4.5.